Mechanical and moisture-related properties of selected dried tempera paints

Abstract

AbstractMechanical properties—modulus of elasticity and strain at break, water vapour sorption, and hygroscopic expansion of selected egg tempera and distemper paints were determined as a function of relative humidity (RH) filling in this way a critical gap in the knowledge required for the analysis of fracturing processes in paintings. The experimental work was made possible by the preparation of several tempera paints, mimicking the historical materials, in the form of large specimens. Lead white, azurite, and yellow ochre were selected as pigments, and egg yolk and rabbit skin glue as binding media. The water vapour sorption and the moisture-related swelling of the paints were dominated by the proteinaceous components of the binders. The linear hygroscopic expansion coefficient of the dried egg yolk binder was approximately 1 × 10−4 per 1% RH, several times less than the coefficient of the collagen glue (4 × 10−4 per 1% RH). The moduli of elasticity of egg tempera paints at the RH mid-range were comparable to the moduli of aged oil paints, whilst the modulus of elasticity of the distemper paint was close to values measured for animal glue-based grounds. The paints experienced the transition from brittle to ductile states on increasing RH. The egg tempera paints were found to be more brittle than the distemper paint, gessoes, and, generally, aged oil paints. The observations modify a frequently used laminar model of panel paintings in which the mismatch in the response of glue-based ground layer and wood substrate to variations in RH has been identified as the worst-case condition for the fracturing of the entire pictorial layer. This study demonstrated that tempera could be more brittle than the ground layer and in consequence more vulnerable to cracking.